FIELD OF THE INVENTION
This invention relates generally to improvements in methods and apparatus for forming three-dimensional objects from a fluid medium and, more particularly, to a new and improved stereolithography system involving the application of enhanced data manipulation and lithographic techniques to production of three-dimensional objects, whereby such objects can be formed more rapidly, reliably, accurately and economically, and with reduced stress and curl.
It is common practice in the production of plastic parts and the like to first design such a part and then painstakingly produce a prototype of the part, all involving considerable time, effort and expense. The design is then reviewed and, oftentimes, the laborious process is again and again repeated until the design has been optimized. After design optimatization, the next step is production. Most production plastic parts are injection molded. Since the design time and tooling costs are very high, plastic parts are usually only practical in high volume production. While other processes are available for the production of plastic parts, including direct machine work, vacuum-forming and direct forming, such methods are typically only cost effective for short run production, and the parts produced are usually inferior in quality to molded parts.
Very sophisticated techniques have been developed in the past for generating three-dimensional objects within a fluid medium which is selectively cured by beams of radiation brought to selective focus at prescribed intersection points within the three-dimensional volume of the fluid medium. Typical of such three-dimensional systems are those described in U.S. Pat. Nos. 4,041,476; 4,078,229; 4,238,840 and 4,288,861. All of these systems rely upon the buildup of synergistic energization at selected points deep within the fluid volume, to the exclusion of all other points in the fluid volume. Unfortunately, however, such three-dimensional forming systems face a number of problems with regard to resolution and exposure control. The loss of radiation intensity and image forming resolution of the focused spots as the intersections move deeper into the fluid medium create rather obvious complex control situations. Absorption, diffusion, dispersion and diffraction all contribute to the difficulties of working deep within the fluid medium on an economical and reliable basis.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "Apparatus For Production Of Three-Dimensional Objects By Stereolithography", which is hereby fully incorporated by reference herein as though set forth in full, have come into use. Basically, stereolithography is a method for automatically building complex plastic parts by successively printing cross-sections of photopolymer (such as liquid plastic) on top of each other until all of the thin layers are joined together to form a whole part. With this technology, the parts are literally grown in a vat of liquid plastic. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes.
Photocurable polymers change from liquid to solid in the presence of light and their photospeed with ultraviolet light (UV) is fast enough to make them practical model building materials. The material that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. An ultraviolet laser generates a small intense spot of UV. This spot is moved across the liquid surface with a galvanometer mirror X-Y scanner. The scanner is driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique.
The laser scanner, the photopolymer vat and the elevator along with a controlling computer combine together to form a stereolithography apparatus, referred to as "SLA". An SLA is programmed to automatically make a plastic part by drawing a cross section at a time, and building the part up layer by layer.
Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross sectional patterns, there is a natural data link to CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, stress, curl and other distortions, as well as resolution, accuracy and difficulties in producing certain object shapes.
Objects made using stereolithography tend to distort when the materials used change density between the liquid state and the solid state. Density change causes material shrinkage or expansion, and this generates stress as a part is formed in a way to "curl" lower layers or adjacent structure, giving an overall distortion. Materials with less density change exhibit less curl, but many materials that are otherwise useful for stereolithography have high shrinkage. The "curl" effect limits the accuracy of the object formation by stereolithography. This invention provides ways to eliminate or reduce the "curl" effect.
Material shrinkage is a common problem with polymer materials, and with fabrication methods (such as plastic molding) that use these materials. However, stereolithography is a new technology, and the problems associated with distortion due to shrinkage have not been widely addressed. The other main approaches to reducing object distortion taken by the inventors have been to use photopolymer materials that have less shrinkage and produce less stress, or materials that are less rigid and are less capable of propagating strain.
These other methods are somewhat effective, but have disadvantages. The earliest way t o achieve low shrinkage in a photopolymer was to use oligomeric materials with high initial equivalent weights. These materials shrink less because there is less new bond formation per unit volume in the photo-initiated polymer reaction. However, these high equivalent weight materials generally have higher molecular weights and much higher viscosity at a given temperature than the lower molecular weight materials. The high viscosity leads to slow leveling of the liquid surface. The high viscosity can be overcome by using increased process temperature, but higher temperatures limit the liquid lifetime.
The shrinkage in photopolymers is due to the shrinkage in the formation of the acrylic bonds. Photopolymers can be made by reacting other functional groups than acrylics, but they have substantially less reactivity than the acrylic bonded materials, resulting in generally inadequate speeds of solid material formation.
Materials that are somewhat flexible when formed usually produce objects with less distortion, since they cannot transmit strain long distances through the object. However, this property is a disadvantage if the goal is to make stiff objects. Some materials are soft when formed, and then harden when post cured with higher levels of radiation or other means. These are useful materials for stereolithography. The whole subject of materials that produce less distortion, because of the way they make the transition from liquid to solid, is currently being studied. However, materials do not currently exist which produce distortion free parts.
There continues to be a long existing need in the design and production arts for the capability of rapidly and reliably moving from the design stage to the prototype stage and to ultimate production, particularly moving directly from the computer designs for such plastic parts to virtually immediate prototypes and the facility for large scale production on an economical and automatic basis.
Accordingly, those concerned with the development and production of three-dimensional plastic objects and the like have long recognized the desirability for further improvement in more rapid, reliable, economical and automatic means which would facilitate quickly moving from a design stage to the prototype stage and to production, while avoiding the complicated focusing, alignment and exposure problems of the prior art three-dimensional production systems. The present invention clearly fulfills all of these needs.